The invention relates to lug assemblies and, more particularly, to a drilled hole composite lug assembly typically used in joints such as clevis type joints, for example in the main rotor cuff of a helicopter.
Drilled hole composite lugs are commonly used in clevis type joints, where a load is transferred from one member to another by a bolt or pin. It may be desirable to bush theses composite lugs to provide protection against damage to the laminated lug structure which might occur during bolt installation and/or removal, or due to normal wear due to high bearing pressures and small relative movements of the mating parts.
Thick-walled bushings are often used to repair mis-located holes in expensive composite parts. These bushings can be installed in an oversized, mis-located hole and then drilled in the correct location.
In lugs for joints where loads are transmitted in a direction parallel to the bolt axis, it is frequently desirable to incorporate shoulder bushings which can provide protection for the faces of the lug. Shoulders can be bonded in instances where the lug bearing stress does not exceed 24,000+/−4,000 psi which is the typical adhesive bearing allowable stress.
This allowable stress applies to both loading parallel to the bolt axis (T) and loading perpendicular to the bolt axis (P). For the loading parallel to the bolt axis, the bearing stress is T/A, where A is the contact area between the bushing shoulder and the composite part. For loading perpendicular to the bolt axis, the bearing stress is P/Dt, where D is the diameter of the bushing “shank”, and t is the thickness of the composite laminate. The bearing stresses due to loading parallel to the bolt axis are always compressive, and a stress level above the allowable will produce breakdown of the adhesive bond. For loading perpendicular to the bolt axis, bearing stresses are compressive in the loaded direction, and tensile on the unloaded side of the bushing. When the bearing stress is exceeded, breakdown of the bond on the loaded side of the bushing will occur. An additional failure mode which often occurs is a tensile failure on the unloaded side of the bushing. This mode has a much lower allowable. Composite bushings with relatively low modulus (i.e. +/−45 degree fiber orientation) are relatively compliant, and are able to deform with the loaded hole, with reduced tension stresses.
Composite components having composite drilled hole lugs, for example graphite laminated composites, are designed for bearing stresses which are up to 24,000+/−15,000 psi, which therefore render these components outside of the useful operating parameters of a normally bonded shoulder bushing. In order to reduce the bearing stress, the composite lugs would need to be increased in diameter and thickness to reduce the bearing stresses by nearly a factor of 4 and for many applications this results in an undesirable weight penalty.
With the foregoing in mind, it is the primary object of the present invention to provide a composite lug assembly including a structural bushing application which is well adapted to highly loaded composite lugs.
It is a further object of the present invention to provide such a lug assembly including a shoulder bushing which avoids the foregoing problems.
Other objects and advantages of the present invention will appear herein below.